I tried taking photos of books that I didn't think I wanted to keep anymore but did want to be reminded of. I got rid of the books but then whenever I saw the photos of the books in my phone I found myself momentarily regretting that I didn't have them anymore.
I've taken a similar approach with the opposite effect. With books though I can understand where you are coming from.
If I was you I would look to getting rid of other things so you can keep your books or I would impose some artificial rule like "if I haven't read 10 or more pages in 3 years the book has to go."
I've pared down my books to owning only timeless reference books or grander "coffee table" style volumes. I find people who would like to borrow the rest, and tell them "No, but you can have it if you promise to give it to someone else once you're done with it". The idea of a book flowing between lots of people makes you feel good and masks the loss of not owning it.
